The global adoption of mobile phones has prompted development of small, high-resolution color displays to handle the increasingly stringent audiovisual needs of these devices.
Despite shrinking to less than the size of a deck of playing cards, mobile phones have become fully functioned devices that are not only able to make and receive calls but also are considered e-mail, calculator, schedule manager and alarm clock to be their basic functions. Furthermore, these phones can do digital still photography and provide photo transfer, gaming, storage, music playback, web browsing and mobile TV.
High-resolution color displays are helping phone makers incorporate more visual functions into the mobile handset since the quality of the image plays a major role in determining the perceived quality of the overall device.
Liquid crystal displays for mobile phones continue to improve. New LCD cell structures can increase image contrast, which can make dramatic improvements in the quality of color images. In order to reproduce crisp moving images, LCD panels with faster response time are being developed by using material improvements and new controlling software. As a result, displays will continue to improve so as to meet the increasing consumer demands for bright, detailed colorful images and high-content information from their mobile phones. Simpler phones can get by with a transflective LCD, which is adequate for simple indoor and outdoor functions. But audiovisual content is driving the movement toward transmissive displays with superior image quality for still and moving images. These displays deliver higher brightness and higher indoor contrast along with more-vivid colors and wider viewing angles.
Today, 2.2 inch diagonal displays are standard for mobile phones, either in QCIF (176×220) or QVGA (240×320). The trend is toward larger screens with 2.4 inch diagonal size and VGA to create more-detailed images. It is unlikely that the displays for the mobile phones will get any larger than this, at least for the near future, since a lager display will make the phone too big to be convenient. It is also unlikely that the resolutions of the displays higher than VGA will be required. A 2.4 inch VGA screen has 333 pixels per inch, which is more than three times as many as a typical SXGA desktop PC monitor.
One problem with the increased resolution is that the LCD cell aperture ratio is decreased, so a brighter backlight is needed to achieve the same image brightness as that of a lower resolution panel. For example, a 2.2-inch QCIF+ panel has a 60 percent aperture ratio, compared with only 10 percent for a 2.4 inch VGA panel.
While consumers demand lager, higher-resolution displays on their phones with brighter images, they also want their phones to be thinner and lighter. This means not only the displays must be thin (a typical 2.2-inch QVGA panels is only 2.6 mm think, including the backlight) but also the supporting circuitry must be small. The typical large-scale-integration driver chips are 2 mm thick, but new designs have halved that dimension.
Power consumption is another important issue for display design. Brighter backlights and more functions require more power, yet batteries take up more space and add more weight as the storage capacity increases. As a result, power savings must be designed into every aspect of the device. The LCD panels of the typical 2.2 in QVGA display consumes only 12 milliwatts, but the LED backlight draws an additional 216 mw, therefore the current challenge is to reduce the power consumption of the lighting system.
Unfortunately, today's LED backlighting system cannot ease the above-mentioned problem. Almost all the full color mobile phone cannot be read adequately under the sunshine no matter how powerful the battery is. Most currently available cellular phone devices are not suitable for outdoor applications due to the sun shine washout effect. The intensity of the incident sunlight can be many times more than the luminance of the display's built-in back lighting. The intensity of surface reflection of the sunlight is so high, for instance, more than 200 nits, which will substantially washout the displays image. People usually cannot recognize the information on the display. For instances, in the sunshine people can even hardly find the phone number on the screen to dial. On the other hand, the back-lit panel attached behind the display panel is the major aspect of the power consumption in both indoor and outdoor applications. Generally, more than half of the electric energy of a device is consumed by the LED backlighting system, which requires a bulky battery structure to ensure an enough operation time. Obviously, how to realize longer operation time is always the big challenge for a mobile phone design.
In the US patent application with the U.S. Ser. No. 11/788,096, the applicant has described a sunlight readable direct-view and projection view computer device, herein incorporated by reference.